you are right not to trust on the benefits. i've personally and I've known many others who have retired from employers (civil service in my case) that hired in and worked under what were supposed to be iron clad post retirement benefit guarantees restated year after year for decades in union contracts that have been taken away. sometimes a court battle (in my former employer's case) gets some clawed back but then they can be SO freaking tricky it prevents the retirees from actually benefiting.
one of the most common i've seen in several states is that the employer will only offer ONE plan that covers retirees that live 'out of region' (it can be a small radius region from the employer's location). they don't care what the premium is b/c they more often than not are capped with a dollar amount they contribute or only a percentage for retirees. if they can make it unaffordable/unusable (no preferred providers in many states or at least none that have taken new patients for years) then the retiree opts out and the employer saves money. I was curious to see what my former is charging for 2026 so I just looked (right now is a great time to check up on this stuff for the future b/c open enrollment stuff is online for active employees and retirees on most employer portals)-

they still only offer one plan out of region, it's a Medicare supplement (they require you to apply for and take Medicare when you hit the age and are retired)-over $2000 for a couple who are both on Medicare
*these are monthly premiums*, but increase that to $5000 if one of the spouse's isn't on Medicare yet and needs traditional insurance. obscene.