Reiser4 developer Edward Shishkin did not release a patch for chachictetl, i.e., 'kernel', 5.15, but Metztli Reiser4 patch for 5.15.220 Long Term Support (LTS) is available here, as well as proof-of-concept reiser4 software format release number (SFRN) 4.0.2 build asymptotically approaching the 'Debian way' development paradigm. As a matter of fact, Mr. Shishkin's reiser4 patches skipped the 5.15.xy altogether, as he released reiser4 for 5.13 and subsequently for 5.16 only. Back when I was using Github, I observed Mr. Shishkin's advice to a reiser4 user interested in reiser4 -enhancing 5.15:

Assuming you need a stable [reiser4] release (format 4.X.Y) I would recommend you to patch vanilla 5.15 kernel with reiser4-for-5.13

And, indeed, his advice initially worked. Nevertheless,

Ah mochipa Tlalticpac: zan achica ye nican ≈

«Не вечно на Земле: лишь краткий [мимолетный миг] здесь». ≈

'Not forever on Earth: only a little [fleeting moment] here'

since, as the 5.15 long term support kicked-in, and advanced, some kernel code was backported from upstream, and then the reiser4 for 5.13 patch application onto 5.15 became somewhat obsolete and the build halted with error(s), especially since:

fs/reiser4/plugin/file/cryptcompress.c

fs/reiser4/plugin/item/extent_file_ops.c

fs/reiser4/plugin/item/tail.c

required to be updated, as in reiser4 for 5.16 patch Mr. Shishkin had already done. Otherwise, reiterating, the kernel build fails during the 5.15.xy LTS build... continue reading at Metztli IT's Tonalpohualli:

Metztli Reiser4 chachictetl macuilli.caxtolli.matlactli_om_cempohualli, i.e., kernel 5.15.220, LTS