For that route, I think your interrail is valid. The SNCF website sells tickets for it.

The name says it itself, TER Bus. TER = Train Express Regional, so it's a rail replacement bus on which your train ticket should be valid. The line closed after floodings, in 2018.  

If someone should say it's not valid, just insist that it's a valid SNCF ticket. And if that doesn't work, the ticket isn't expensive: 2,5 EUR. 

Since the last few yrs the regions in FR are reorganising their own busnetworks-and merging the old TER lines into it-and info on validity of passes may thus be old and not valid anymore. So be prepared to pay-and as above-in general most are quite cheap, sometimes just 1€. Also very infrequent.

If you board in a Q you will usually see what other people are holding-if thats also railtix then it should work.

Note that SNCF as such does not run any buses-they are all contratced out to local companies, and these may also very well run lines for the region.

OTOH in Germany by far the biggest bus comp. is also DB-all over the old West-part of country, but none of these will take your pass. But soon there will be a 1-month season for all of DE, train and bus-tram, for just 49€. More or less same in DK with DSB, but no cheap season there.

Validity in French rail replacement bus lines varies per region. Yesterday I used a TER bus line in the Grand Est region (St. Dié - Sélestat) and although the onboard scanner didn't recognise the QR code, the driver took a look, scrolled down to the listed trains and then was OK with it. It may have helped that I indeed mentioned it was a valid SNCF ticket.
Normal fare would have been €4 IIRC. But since the rail planner app lists it as an option where the pass is valid, I'd have claimed it back from Interrail if I had to pay.