What
The check controlling background session prewarm bursts tested the delay where it meant to test the concurrency limit. The two settings are still fetched remotely, falling back to 15000 ms between bursts and 3 sessions at a time.
Evidence
tengu_bg_prewarm_burst_concurrency
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
Feature flag
tengu_bg_prewarm_burst_concurrency Not enough to sayNothing here resolved what this flag was doing on this version, so nothing here should be read as on or off.
This account: no value returned · anonymous baseline: no value returned · compiled default in v2.1.248: not a boolean we can read
Read once, for one account on one subscription tier, against v2.1.248. It isn't a statement about your account. What a flag value here can and cannot tell you