One of my past employers in the UK added to the policy all the software the employee writes during the employment (eg. during the weekend, on the personal hardware), is owned by the company.
Several software engineers left, several didn't sign it.
Yes, company was very toxic apart of that. Yeah, I should name and shame but I won't be doxxing myself.
Many years ago an employer tried to to that and everyone .. just refused to sign the new contracts. The whole thing sat in standoff limbo for months until the dotcom crash happened and the issue became moot when we were all made redundant.
This is what I've done in those rare cases I've had to fix a bug in a tool or a library I've used professionally. I've also made sure to do that using online identities with no connection to my employer so that any small positive publicity for the contribution lands on my own CV instead of the bureaucratic company getting the bragging rights.
Even at places that are permissive about hobby code, a company ought to want to put its name on open source contributions. These build awareness in the programming community of the company and can possibly serve as a channel for recruitment leads. But the (usually false) perception of legal risk and misguided ideas about what constitutes productivity usually sink any attempts.
It is amazing how companies want this "marketing" but don't want to put the actual effort to make it possible.
A tech company I worked at once had a "sponsorship fund" to "sponsor causes" that employees wanted, it was actually good money but a drop in the bucket for a company. A lot of employees voted for sponsoring Vue.js, which is what we used. Eventually, after months of silence, legal/finance decided it was too much work.
But hey it wasn't an exception. The local animal shelter was the second most voted and legal/finance also couldn't figure it out how to donate.
In the end the money went to nowhere.
The only "developer marketing" they were doing was sending me in my free time to do panels with other developers in local universities and conferences. Of course it was unpaid, but in return I used it to get another job.
