Can Time Machine Keep Milestone Backups?

No. Time Machine automatically makes hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for all previous months. The oldest backups are deleted when your backup disk is full. A user can backup to multiple drives and could retain older backups if they really felt that they needed to; although that seems wholly unnecessary.

muguy is absolutely right and be guided by that.

Advise your client that what she is requesting isn't a backup but an archive, and they are very different things.

Apple says here about Time Machine backups that the purpose is (and pardon the reproduction of the split infinitive): to automatically back up your personal data, including apps, music, photos, email, and documents. Having a backup allows you to recover files that you later delete or can't access. So it is to save for a limited time for later recovery after loss - or to move an existing system to another Mac.

Keeping a history of old data serves another purpose which is to preserve specific types of data for a longer period.

If she wants to keep an archive of emails, photos, music or so on then there are other ways of doing it and usually to an external drive - or more correctly several copies of it. One digital copy is rather perilous and as risky as having only the primary version.