I believe the Chartres is still in service in the Mediterranean - and she's probably not the only one.
To add further detail then, which of these survivors are still plying their original route? The remaining Isle of Wight trio, and I presume the Windermere vessels?
There are no Sealink era vessels still plying the Isle of Wight routes; the oldest currently in service - St Cecilia - was built in 1987, three years after the demise of Sealink as such.
The UK part of Sealink became Sealink British Ferries upon privatisation in 1984, but the SNCF part of the business was split off at that point, so it probably comes down to how we are going to define Sealink itself?
Are we going to count "Sealink Stena Line" as well?
If so, what about when they later reversed that to become "Stena Sealink Line"? Surely by that point it was arguably effectively just a brand and bore no relation to Sealink the business?
There's only the St Cecilia left from the 1980s fleet, though.