Classic is core theme. If it's been properly removed assume it won't matter. If not properly removed, potential for issues in future git pulls? Dunno ... neither of us have been down that road yet and am not a developer nor a volunteer tester of the next major release 3.8dev - just an 'omicron' tester ... ie, typical user.
Don't think Moodle plugins site has core themes available for downloading/installing separately.
My git test directory has all versions from 2.0 up to 3.7.
find ./moodle??/theme/ -name classic
finds only one ...
./moodle37/theme//classic
./moodle37/theme//classic/scss/classic
that of moodle37
Guess you could acquire 37 code to a git acquired sideload directory, then copy the theme/classic directory to your production site and install it that way.
'SoS', Ken