finding (or alternatives) 16GB 3866MHz DDR4 SDRAM UDIMMs?

Can you still buy 4x16GB 3866MHz DDR4 SDRAM UDIMMs, such as two G.Skill 
F4-3866C18D-32GTZR preferably without horrible 'RGB' LEDs (though if only 
option and OpenRGB can turn off, we'd use this model)?  I searched Google, 
New Egg, Amazon, Best Buy, Ali Express, but closest are 3733MHz, 4000MHz.  
We want to upgrade old PC--GigaByte GA-Z270-HD3P--with fastest RAM, which 
GigaByte.com says is 3866MHz... can you even run 4000MHz at all--stable at 
3866MHz--or best switch down to 3733, 3666, or 3600MHz, etc.?

Internet Relay Chat (IRC) gave unhelpful replies like 'use DDR5'--GA-Z270-
HD3P can't--and someone saying that technology generation doesn't 'scale' 
well with RAM (whatever that means) but we're not getting rid of it nor 
using slower just because of vague/subject-changing IRC statements.

Of course, if/when there are decent/better DDR5 SDRAM system-/logic-/
main-/mother-board options (see my 2024-1-4 alt.comp.hardware.pc-homebuilt 
thread asking about CPUs) I'll build a new PC (for me, not replacing GA-
Z270-HD3P (family PC) so saying 'use DDR5' is irrelevant)... that is, if/
when a new DDR5 full/extended ATX system-board has plain PCI slot(s) and 
preferably can run powerful AMD Ryzen (7950X(3D) Zen4 or successor Zen5) 
or maybe Intel Core i9 almost as powerful (threads) for computer 
programming/science (particularly compiling large projects, which still, 
primarily needing larger number of threads, only AMD is best at)... 
entirely unrelated to upgrading old PC for the many years we plan to use.