Do they have to be stamped MF or just reach the criteria, certainly the Wago box's are not stamped, but do reach the standard, but then are they meant to be maintenance free, the lighting junction box's for downlighter's in a ceiling for instance can be pulled down to change a downlighter and none of the one's I have available are stamped MF.
Seems it was added at the last amendment to 526.3.
526.3 Every connection shall be accessible for inspection, testing and maintenance except for the following:
(i) A joint designed to be buried in the ground
(ii) A compound-filled or encapsulated joint
(iii) A connection between a cold tail and the heating element as in ceiling heating, floor heating or a trace heating system
(iv) A joint made by welding, soldering, brazing or appropriate compression tool
(v) Joints or connections made in equipment by the manufacturer of the product and not intended to be inspected or maintained
(vi) Equipment complying with BS 5733 for maintenance-free accessory and marked with the symbol and installed in accordance with the manufacturer’s instructions.