Forum

According to several IEEE documents, including 802.11 (2007) section 11.1.3:

"Active scanning is prohibited in some frequency bands and regulatory domains."

Does anyone know where (as in what countries or regulatory domains) this might be true in the 2.4 GHz range?

I would assume that this is only a concern where DFS rules apply in the 5GHZ range.

Yes it applies to all DFS channels in ETSI and possible also in FCC domain.
This stops a VoWiFI handset to use active scanning when the candidates in the roaming table are not fullfilling the roaming criteria, thats being stronger than the one you curently are on, so the handset would like to probe as a last resort.

Thus to achieve high roaming performace you are stuck with UNII-1 or UNII-3 (not allowed in ETSI) which reduces your channel selections dramatically. And in th ecas of using 40MHz channels it is even worse.