From Acoustic 2025

The intention behind the master suppression list is that it suppresses emails across ALL of the databases that an organization has. Emails on the suppression list come from:

• Direct to Acoustic - ie some emails are suppressed behind the scenes by Acoustic, across all their customers. (This used to be the case with AOL emails) Additionally some contacts opt-out directly with Acoustic (for example by by replying unsubscribe or marking as junk)
• From CiviCRM - ie we do a nightly upload to add (but not remove) from the master suppression list. Per earlier notes this upload includes not only opt outs but emails that are not the ‘preferred’ email

The master suppression list is intended by Acoustic to be used for contacts who have opted out but we over-use it by adding former addresses to it - because that way we can be sure the contacts are not included in emails (ie they are always excluded and the Acoustic user does not need to remember to exclude them).
However this has the consequence of the suppression persisting when the email becomes appropriate to use again (e.g because they do not choose to opt in and then next time they do choose to opt in).
Historically we have dealt with this by intermittently purging the master suppression list and re-creating it from current Civi data. While this has been fine as a stop gap, there are signs it's not a stable process and we are losing an unknown number of usable emails.