Effect of a one leg connection, is that attenuation doubles, so resulting connection very much depends on the original link. Or to put it another way, some people may not really notice, if its mainly email and a bit of news/weather.
It is likely the problem is in the exchange, and is a failed migration to TalkTalk, i.e. they have removed the PSTN side, but left the ADSL side going to the BT Wholesale hardware, so speeds would be normal.
i.e. Openreach has messed up. If the line had moved to TalkTalk the broadband with Plusnet would have stopped working