I like planning some of these things as parts of projects.

To do X feature, you need to clean up tech debt Y.

Tying it to projects means we can intentionally distribute these to different team members.

... It also means that if cleaning up the tech debt is hard, you're project gets delayed for something where you could have found a way around fixing the tech debt.

My own productivity has been horrible lately, so maybe that should be taken with a grain of salt:P