I was wondering how to go about asking for a retroactive forbearance, and how long ago they can go back? I have some late payments from 2018/2019 and I think I had forbearance right before and right after (I had forgotten to renew) but I don't know how to check or find out? My loans are Fed loans with Navient if it matters.
You call them, explain the situation, and ask. How long they will go back is likely company/servicer policy.
Please note that retroactive forebearance does not mean that they will remove the lates from your credit report. It affects your account (whether or not you're past due), but does not require an update to the CRAs.
Let us know if you decide to go down this trail. I learned about retroactive forebearance after ACS messed up my reporting. Now that they're defunct, it seems I have no recourse.