Errata patches for the kernel have been released for OpenBSD 6.7. mmap can exhaust kernel memory for PROT_NONE MAP_SHARED mappings.
Binary updates for the amd64, i386, and arm64 platforms are available via the syspatch utility. Source code patches can be found on the errata page: https://www.openbsd.org/errata67.html As these affect the kernel, a reboot will be needed after patching.