projects are left without maintainers

I don't know how it works in Russia, but in Common Law countries, copyright of work done by an employee belongs to the employer. When you submit code, you make a statement of whose copyright it is, I believe. You need permission from the copyright owner to submit. You not actually submitting your code, you submit code belonging to the copyright owner.

If someone was to contribute code they wrote outside of their employment with a sanctioned firm, the copyright of the submission would vest with the contributor, not the employee (depending on the employment contract and the nature of the submission). The changed ownership of copyright is a significant difference, and probably significant regarding sanctions. Likewise if employment changes.