Depends on what counts as “customizable”.
If flashing firmware from a computer is allowed, there are already a bunch of fully-customizable flashlights. I made a UI toolkit and some example UIs which can be used on a variety of different hardware, and people can customize it to pretty much any UI they can imagine. If the UI can be described in the form of a flowchart, it’s probably possible. The examples include an Olight UI, a Zebralight UI, a Werner-style UI, Anduril, and a couple simplified forks of Anduril.
If customization is limited to config data and not allowed to modify the firmware, something like the FLEX Asgard might work, since it’s configurable over bluetooth. Or perhaps a Lux-RC, though its UI is very small and limited.
If customization isn’t allowed to involve a computer at all, like if it must be entirely built into the flashlight, something like the YLP Unicorn may be of interest. It has a couple base UIs and allows the user to configure what most button presses do. Or a H17F allows configuring each of several modes and options. Or Bistro is pretty configurable.
My recommendation would be to use the most flexible option, creating completely custom UIs and flashing firmware to something like a D4v2 with its optional flashing kit. But I recognize not everyone wants to customize things that extensively, so simpler options are good too.