For example my Samsung 35E doesn’t work with BLF Q8, Folomov EDC-C4, Haikelite MT09R, Astrolux MF-02. Because they all require button top.
OTOH button top Lumintop LM34C works at least with Folomov (haven’t checked with any other of those), but doesn’t work with Emisars.

There are no brand 26650 cells, so one can’t follow your advice. And it happens that the best cell about this size is actually slightly larger than standard and doesn’t fit all lights, regardless if you buy with button or without.

21700 is becoming another standard. And buyers realize that the extra 5 mm in length means that not all chargers can work with them.
There’s 20700 also which largely overlaps with 21700, but is compatible only one way. Some manufacturers go with 20700, some 21700….this is true both with cell and light manufacturers.

I haven’t yet seen “Protected 21700”. It will be interesting when they arrive…

I’d like to note that I wouldn’t use Samsung 35E in Emisar D4 anyway though - because its medium discharge construction is not a good match for this light. But this incompatibility is actually a part of the fun - you tweak component selection to get the best results. Not to work around needless limitations.

I love doing research. But research of intrinsic properties like capacity, internal resistance, low temperature performance, cycle life etc.
Not of “cell X is .1 mm too wide to fit light Y”.