Source: http://www.celestrak.org/
(Please pardon the Bluesky links below)

*URGENT MESSAGE:* From TS Kelso of CelesTrak “We [The US Space Force 18th
Space Defense Squadron] will run out of 5-digit catalog numbers at 69999
not 99999, which is estimated to occur around 2026‑07‑20 (we’re currently
at 68014). At that point, newly cataloged objects will have 6-digit catalog
numbers of 100000+ and GP data will not be available for them using the TLE
format. CelesTrak developed new formats that removed this limitation (and
finally fixed the Y2K problem) in May 2020 and immediately began providing
GP data in those formats for software developers. The same limitations
apply to the legacy fixed-field SATCAT. Follow @TSKelso on Bluesky for the
latest updates, tutorials, and changes as we approach this transition.
Spread the word!”