IMO to use the term renege doesn't matter whether it was a promise or a contract. You are right that in most cases retiree medical is a promise and not a contract... but there is less recourse to go back on a promise like retiree medical... if you go back on a contract the other party can sue you for damages or specific performance.
The problem often comes with documenting the promise. Was it actually printed in an employee HR or benefits manual? Did the wording imply it was an ongoing, earned benefit or only describe how it worked at that time? Doing something for people is not a promise that you'll continue doing it.
At the Mega where I toiled, post 65 medical benefits were given in two tiers depending of your original hire date. Recently, they reduced this to a single tier, eliminating the higher (more $) tier and moving those folks to the lower tier. Rumors are now spreading that the lower tier will be removed soon.
The older guys at my Friday morning breakfast group who were on the higher tier were plenty grouchy when it happened. But all readily admitted they knew the whole retiree medical plan was voluntary and at the discretion of the company. No one, including me, could come up with any documentation showing it was an earned benefit (like our DBP's). No law suites emerged. One guy said he was seeing a labor attorney, but has never come back with anything.
So, maybe it was a promise, or maybe it was something they were doing for a while and now stopped doing. And there was never a "promise" of an ongoing benefit. Everyone readily admitted knowing Mega could stop the program at any time.
It seemed a lot like when Mega discontinued our bonus system which had been quite lucrative for a while. Since they used the same formula for years, it seemed "promised" that that's how we'd always get incremental pay in profitable years. Then it was gone. I felt reneged on. But there was indeed nothing that said they were obligated to go on with that bonus formula/system forever.
But if it makes OP or others feel better, by all means use negative terminology about all these negatively impactful business decisions. It's likely therapeutic. I know my geezer work buddies at Friday breakfast sure seemed to feel better after we spent numerous mornings bitching about it. And it still comes up from time to time. Just assume that an activity of one time set a precedence that morphed into a promise of eternal continuation and its current elimination is a renege. I get it.