Alternatively if you have uBlock and are scared of one-off extensions, you can add this generic modal filter[0] list from the webannoyances repo.

I found this by searching "modal" on filterlists.com and clicking the subscribe button. IMO there should be a Medium-specific one like they have for stackoverflow or youtube.

I cannot personally vouch if the modal filters are overzealous. I'd rather avoid Medium than putting up with this sort of thing.

[0] https://raw.githubusercontent.com/yourduskquibbles/webannoya...