In any Writer document, hit ^f to search. Search bar has a MatchCase option with M underlined but ineffective: ~m does nothing; only a mouse click toggles it.
Confirmed on Windows 10 Ent 64-bit en-US (1803) with
Version: 18.104.22.168.alpha0+ (x86)
Build ID: a675fe37abc134cc5cbf54385ca99c8d1417ce2b
CPU threads: 8; OS: Windows 10.0; UI render: GL; VCL: win;
TinderBox: Win-x86@42, Branch:master, Time: 2019-07-26_09:09:34
Locale: en-US (en_US); UI-Language: en-US
IIUC the checkbox widget comes from the <Ctr>+H Find&Replace dialog, there the widget correctly responds to its accelerator. So, guess the the issue is difference in checkbox controls between dialog and toolbar instances.
The <Tab> to position, and <Space> to select the checkbox works--so not locked out of the tool.
Apparently Match Case widget in the Find Bar is an odd tb control (ref 114557).