The Pymatuning Spillway Trail appears to have been expanded to South Chestnut Street, as described in We Conserve PA's article, and the bridge has been reconstructed. In addition, the new DCNR map for Pymatuning State Park shows that the multi-use trail does extend to South Chestnut Street. I think that perhaps you may have taken a wrong turn. You are correct that you can't access the trail from the senior center, but there is a paved part of the trail that goes east of Hartstown Road/South Mercer Street farther south from the senior center, and you will pass by fairly close to their parking lot (although you would have to hop a fence and trespass through their yard to get to the parking lot from the trail). The Pymatuning Spillway Trail seems to be correct as it is on OpenStreetMap in November 2024, although the western off-road part from the DCNR map is not mapped yet. The trail is incorrect on Google Maps, but we cannot do anything about that here.
https://weconservepa.org/blog/pymatuning-state-park-trail-extension-honored/
https://elibrary.dcnr.pa.gov/GetDocument?docId=1737310&DocName=PYMA_ParkMap.pdf