You could buy from a provider that advertises non-use of DRM like GOG, or on Steam, it lists third party DRM, so you can know whether you have the tools to remove it (and whether you have the tools to remove Steam's DRM, or whether the game appears on a web list of games that don't use any DRM). You could also refund it if you can't verify you're able to successfully back it up and run the backup on a computer or user session without Steam installed. For multiplayer, if it's possible, you can find people discussing it on the web (maybe in pirate communities). Otherwise, just don't buy it.

Some recent stats indicated most gamers buy at most two games per year, so it's not a ton of work to ensure they have a working archive.

Steam's lawyers would say that one should know by reading the terms of service for the storefront and the purchase. But in the real world, how often does that happen?

This is 90% of the reason I don't bother buying modern computer games. For me, I assume games require phoning home and use some kind of DRM unless it is otherwise advertised.

Assume everything is encrapified with “strong” DRM unless credibly demonstrated otherwise.

The best thing you can do is ask the distributor or publisher. It shows them this is wanted and gets you answers.