GOG Connect may be along the lines of what you're describing. The compatibly of games is on a temporary basis set by the publisher, but, yes, it does allow you to activate Steam purchases on GOG.

If by "port" you mean remove it from a steam accounts library and add it somewhere else on a different platform then no, I never heard about something like that. But you are free to give an extra copy to the other account, while accepting that some people will abuse this. Just like you can let people buy the game somewhere else and give them steam keys that can afaik be generated for free while missing out on some steam features like reviews being counted.

Or do you exclusivity contracts? There are certainly games that were sold on Steam and are now exclusively available on Epic's Store. Satisfactory is an example that comes to mind. Copies sold on steam are still honored there, so the contract probably doesn't allow you to force users off steam after the sale.

In general I'm quite amazed at how little abusive practices Valve has shown while having a quasi monopoly for such a long time. Stagnation is probably the one, if you want to count that. I hope they won't become worse under pressure. And I certainly have little trust in other companies trying to replace Steam to behave as well.