Hey there,
Please understand how the anti-undermesh system works. What it does is it prevents players from existing in undermesh areas. Buildings that have been previously placed will remain in there until decay sets in, but they will be unreachable.
If players can still traverse undermesh locations in the Exiled Lands, please report it to our team via a DM to @Community with either location where this is possible and/or the method to achieve it so we can continue tuning this feateure.
Based on internal telemetry and data gathered since the activation of this feature yesterday, we have observed it is working as intended, with a large number of player characters who have attempted to access undermesh locations being killed in the process.