2026.1 is still supported, and you can pin your installation to it by setting version: release/2026.1 in your app.yml. In that case, running a rebuild would’ve pulled in the security update on the 2026.1 branch.
I guess you were using version: esr? In that case, you are tracking the ‘latest esr’ version, which is now 2026.7.
You can find information on the support periods, and the overlaps between ESR support, on https://releases.discourse.org/
Unfortunately it isn’t possible to downgrade Discourse. But if you’d like to avoid this in future, you can pin to release/2026.7 (and make sure to update that pin manually before 2026.7 drops out of support).