Chaosmancer
Legend
No it's extremely relevant to point out that there are materials like silver that could have been included to bypass damage immune/resist to allow both campaign types to exist without needing to place the onus on the gm of building such a thing for wotc. They included silver as a half measure because they wanted to give nonmagic campaigns something special for certain monsters, but because they were too focused on faerun they forgot to include materials added by eberron & other settings in both the phb & more importantly the monsters in the monster manual (someone mentioned red steel & others earlier).
There is also the fact that it's not simply a matter of having the "right stuff" & it's disingenuous or just pure lack of experience speaking to keep saying it... given how many times this has been covered I'm wondering if it's a deliberate misrepresentation?
Take the . t was not immune to nonadamantine damage, it shaved off the first 10 points of anything that hit it meaning that you eeded to have a heavy hitter like a chaging/power attacking/etc raging barbarian/crit fisher/etc hit it hard or someone else like a build built for reliable average damage to get lucky with the damage rolls leaving those big number builds feeling valued in their choices by getting to show off abilities like charger & such that often took some setup. A gm with lots of reliable low-medium damage number players & no big number ones should probably think twice about using such a creature & instead use creatures those reliable low to mid number damage players an handle more often. That's opposed to the 5e one where you have a 178 hp creature turning into a mindnumbing 356hit point slog of just hitting it again.
so clearly the most logical course of action was to remove any nuance & depth from resistances rather than including a blurb along the lines of a gm running a campaign world with no magic items or extraordinary materials may want to avoid using creatures with resistances & immunities to nonmagical damage or include some mundane way of damaging these creatures such as having weapons blessed by a religious figure or bathing them in a particularly pure stream if they are used." Also does your no magic campaign lack weapon choices that deal bludgeoning piercing or slashing damage that would prevent creatures from being undefeatable if they had resistances & immunities to those?
Please read your own statblock pictures again.
DnD 5e Stone Golem has no damage resistances, so it never becomes a "mindnumbing 356 hit point slog". Actually in this example, the Golem who ignores 10 damage from every attack and has 107 hp could be the longer slog.