Obsolescent
Etymology
From Latin obsolescens, the present participle of obsolescere, "to fall into disuse, wear out, grow old." The present participle is the whole point: it names the ongoing process of becoming obsolete rather than the finished state. That is the useful distinction — "obsolete" is dead, "obsolescent" is dying.
adjective · /ob-suh-LES-uhnt/
In the process of becoming outdated and passing out of use, though not yet fully obsolete.
What it names
The precise word for something on the way out but not yet gone — an obsolescent format or skill still in use while its successor takes over.
Example
The fax machine was obsolescent for years, still humming in a few offices long after everyone assumed it had disappeared.
Related idea
Obsolete marks a thing already superseded and no longer used; obsolescent marks it mid-decline, on the path there but not yet arrived.