Railroading is NOT removing choice, it is removing the impact of character actions on the events in the game. In my experience, restricting mechanical choices actually has the effect of focussing the players on creating something more compelling within the environment presented - some of the best, most compelling, most innovative roleplaying I've seen has come in Basic D&D, where mechanical options were severely limited. Also is a 1E DM 'railroading' when they choose to abide by the class restrictions regarding demi-humans, armour restrictions regarding other classes, alignment restrictions for Paladins, Monks, Rangers and Bards, or adhering to the level caps in that edition? What about Monk trying to use burning oil, a Thief trying to wield a 2-handed sword (the book says they may not), or a Cleric wanting to use an edged weapon (the PHB states they are 'forbidden'), so it's more than simply them not being able to be proficient in them in these cases. It's not railroading if a DM insists on using these restrictions as written.
By choosing to play a Druid, you will have agreed to abide by the no-metal restriction - just as a Thief player in 1E will have agreed to abide by 'no 2 handed swords'. If you then decide to break that agreement, then no, I am not railroading you by denying that choice. You (the player) is simply being difficult, and somewhat disrespectful. If you don't agree to the lore, you don't play the class. If you try to push it, you don't play full stop.
Railroading is not removing choice in selecting mechanics, but rather removing options in the game world by stating the possible to be impossible without any reason given besides not wanting the player to do it, or "the rules". Saying a Magic User cannot effectively use a shield due to their lack of martial training is not railroading, but saying they can't pick it up and strap it to their arm and gain no benefit while accepting the normal encumbrance or any other associated penalties, is. What if that Magic User is strapping it to their arm solely because that makes it easier to carry for the purpose of running it to their Fighter? What if their Fighter died and they want to take the shield back to town with them? Will the universe implode if they equip it? What if they plan to use it for an out of combat purpose, such as using it to Indiana Jones something else of equivalent size and shape off of a pedestal? The same held true of Druids. They couldn't cast magic while wearing metal, but what if they're hiding inside a decorative suit of armor in a castle, waiting alongside their party to perform an ambush? They'll toss the armor off as soon as they perform the ambush, so there'd be no loss.
There's tons of reasons to use items outside of their intended purpose, and it's totally possible to attempt to use something that you're not proficient in using. You won't have high odds of success, because you'll inherently suck with it, but you can darn well try. A character with 8 strength may not be able to lift the rock that the 20 strength Barbarian can, but they can darn well pull their back out trying.
Throughout D&D Druids have been leather armour clad spell casters who share a number of spells with regular clerics, who derive their power from nature, who have the shapechanger ability at some point, and who utilise simple or 'tool' inspired weapons - that IS pretty consistent. In 3 editions they had a neutral component to their alignment, but as 5E (sadly) reduced the impact of alignment mechanics that was lost. Being pedantic about the level they get to shape change proves nothing.
Sure, and the reasons for their limits have changed, as well as the source of their power. Druids used to gain their power solely from nature, now they can draw it from a deity or from nature. Druids lost their magic in 1E if they wore metal armor or used a metal shield, but they never stated it to be taboo; it simply had penalties for doing so, so they obviously opted not to do it. It became a taboo in 2E, but didn't give any penalties as to what happened if they wore it. This was already a major change in the design, since it went from a hindrance to a taboo. In 3E it went back to being a mechanical penalty, where if they wore it they lost access to their magic for as long as they wore it, plus 24 hours. In 4E it was neither a taboo nor did it have penalties, they simply were not proficient by default. In 5E there are again no penalties as it's still a personal choice without mechanical implications beyond proficiency, as it has been since the Druid was released in 4E during early 2009. We are literally 10 years and 2 editions of D&D into Druids having no mechanical penalty for metal armor, unlike 1E, but people will not let go of the 1E fluff. If we can't let that go, then let's start taking away the Cleric's weapons, and let's buff the Druid's spellcasting to be stronger than the Cleric's.
And if you're going to discuss alignment, they were only required to be true neutral in two editions. That's 2/5, or less than half. They've also had absolutely no such restriction for 2/5 editions, but the 2 editions they've had no limits on are the two most modern. Feel free to cling onto the 1978 restrictions, but it's 2019 and the rules have changed.
Even the ability to only have to remain neutral in one element of the character's alignment as opposed to both is actually a very significant change that was made in 3E, but either way, don't blame 5E for the reduced alignment restrictions because they also dropped those in 4E. So again, 2 editions of D&D and over a decade since that change has been made, so it's likely here to stay. In 3.5 a Druid could be good or evil, which is leaps and bounds away from what the traditional Druid's belief systems were in the past two editions before it. Druids were only allowed before to be acolytes of natural order, but that change allowed them to literally be champions of good or evil. Heck, the whole concept of having to be neutral in only one aspect of your character was silly, because that's nothing like being true neutral. Neutral evil is nothing like lawful neutral, which is nothing like neutral good. They realized that was dumb because they practically gave them absolute freedom already, so they later just removed it completely.