I'm guessing this no longer qualifies as "modern," since the last update was in 2018 and is no longer in active development, but I'd like to say that the 32bit version of the Tiny C Compiler by Fabrice Bellard works on Windows 98 SE.

https://www.bellard.org/tcc/

I found out the other day you can use modern clang-cl with the MSVC6 headers and it just works. you can download them from here https://github.com/itsmattkc/MSVC600 or just copy it from an install if you have one handy.

then run (something like) this:

  clang-cl /winsysroot:"" /DWINVER=0x0400 /D_WIN32_WINNT=0x0400 -m32 /GS- -march=i586 -Wno-nonportable-include-path /imsvc"C:\MSVC6\VC98\Include" hello.c -fuse-ld=lld-link /link /SAFESEH:NO /SUBSYSTEM:WINDOWS,4.0 /LIBPATH:"C:\MSVC6\VC98\Lib" user32.lib kernel32.lib msvcrt.lib

I haven't tried it but i saw this a few days ago: https://github.com/crazii/MINGW-toolchains-w9x