Usually, no: obsolete film doesn’t become unrecoverable just because labs stopped offering the process. Most photographic processes were documented, so in principle a determined specialist could often reconstruct or perform them.

Kodachrome is a good example. Its K-14 process was unusually complex, with dyes formed during processing, so it was never practical for ordinary labs or home users the way C-41 or E-6 are. That makes recovery difficult and impractical, not necessarily impossible in principle.

The bigger real-world problem is often the film itself. Latent images on undeveloped film fade over time, sometimes faster than processing knowledge disappears. So an old exposed roll may yield little or nothing even if the chemistry is known.

So the statement is partly true in practice: a “lost masterpiece” may never be seen. But the usual reason is age, fading, and lack of practical processing—not that the film format has become fundamentally unknowable.