"Druids will not wear armor or use shields made of metal." 5e PHB.There are no rules, and there have never been any rules, that say a druid cannot don a suit of metal platemail in order to sneak into a castle. If a DM prevents me from taking that action, he is in fact railroading me by invalidating my decision.
There is no reference to the circumstances of the thing - is it to sneak into a castle, win cosplay, mock the paladin etc. But then they dont list or try to list all the various reasons one may use things.
So, for 5e, if you chose druid, knowing this rule and did not work out the details with your GM, then to me it's not railroading for the GM to hold this rule as applicable.
Do you see an exception in that rule that says "except to sneak"?