Safe to delete old data after pg_upgrade?
They generally compress well.  I'd just tar and compress it and remove the original, and your monitoring system will probably be happy. (And anything trying to use it should start throwing errors once it is no longer there, and then you can fix them). But you should have a bigger-picture data retention policy, and that should govern how long you keep the compressed copy.

At some point enough activity has happened in v10, that if a major problem surfaced you would just have to fix it rather than rolling back to v9.5 and losing the changes made in v10.  At that point, the old copy is pretty useless except for PITR purposes.