The new Spell Resistant feat is a trap

Specifically the "Magic Resistant" portion, which is likely the main reason a player would pick this up.

The 1d6 bonus to saving throws is only for spells and magical effects. Spells are not nearly as common on monster stat blocks as they were in 2014, but I think most experienced players are aware of that at this point. What likely goes under the radar is the fact that nearly nothing any monster does outside of spells counts as a "magical effect" anymore.

Sage Advice confirms that the only effects that count as magical are those created by spells or magic items, or effects explicitly labeled as magical. Spells are already covered separately within Magic Resistant. Magic items are very rarely built into a monster stat block (I can only recall seeing them occasionally on named adventure NPCs). That leaves effects explicitly labeled as magical.

These are all the effects I found that are labeled as magical in 2024 monster stat blocks:

Darkmantle darkness aura. Sahuagin shark telepathy. Harpy luring song. Intellect devourer detect intelligence. Nothic weird insight. Beholder (and various beholder type creatures) eye rays. Banshee detect life. Strigoi stirge telepathy. Galeb duhr animate boulders. Priest of Osybus soul tattoo. Shield guardian bound. Cockatrice regent magical backlash. Gnoll demoniac hunger of Yeenoghu. Treant animate trees. Deep dragon preservation of knowledge. Mind flayer arcanist arcane tentacles. Brass dragon refreshing water. Copper dragon chatty critters. Death tyrant negative energy cone. Harkon Lucas beast spies. Green dragon beast spies. Spirit dragon unearth ruins. Ankhtepot sands of time. Dracolich soul gem. Sphinx of valor roar. Animal lord lordly presence (mired). Various effects that magically return a weapon to the monster.

Of those, the only effects that involve a saving throw are:

Harpy luring song. Nothic weird insight. Beholder eye rays. Cockatrice regent magical backlash. Gnoll demoniac hunger of Yeenoghu. Ankhtepot sands of time. Sphinx of valor roar. Animal lord lordly presence (mired).

You're not likely to run into Ankhtepot outside of a Domains of Dread campaign. Sphinx of valor and Animal lord are CR 17 and 20 respectively, so you're not going to be seeing a ton of those over any given campaign. That leaves only five monster stat block effects labeled as magical that a player would reasonably have a decent chance of running into at some point in a campaign.

So if you looked at this part of the feat and thought "Well monsters don't cast a lot of spells anymore, but there's all those new spell-like abilities that this would work on" – then you fell for the trap.

EDIT: I haven't seen any monster stat blocks from Arcana Unleashed yet, so there may be more saving throw effects labeled magical in there. But based on the tiny amount up to now, I'm not holding my breath.