Since #28195 got merged, all non-test code relating to the legacy txindex (and its migration) has been removed from master.

At this point, feature_txindex_compatibility.py is just testing whether -txindex works successfully when a legacy txindex exists, but as there is no overlap in data structures this feels like a redundant test and I think it can be removed from a future version. Since #28195 is included for v26, I'd suggest we remove this test for v27.

Historical pulls for context:
#13033: introduce new txindex db and migration code
#22626: remove migration code
#23046: add feature_txindex_compatibility.py