Hallelujah, brother, it helped! Now I have restic.exe which is even slightly smaller than the official one. Thank you from the bottom of my heart!
Also I believe the official documentation should be updated as soon as possible, because it is no longer relevant. I followed it step by step using go build way and got the same large EXE (~40 MB) as go install produced earlier. But -tags disable_grpc_modules helped in both cases.
Build log (2025-03-30)
$ ver && go version
Microsoft Windows [Version 6.1.7601]
go version go1.23.6 windows/amd64
rem Contents of restic-go-build.cmd
mkdir restic
cd restic
curl -L https://github.com/restic/restic/releases/download/v0.18.0/restic-0.18.0.tar.gz | busybox tar xz --strip-components=1
set GOOS=windows
set GOARCH=amd64
set CGO_ENABLED=0
go build -ldflags "-s -w" -tags selfupdate -o restic_0.18.0_windows_amd64.exe ./cmd/restic
$ restic_0.18.0_windows_amd64.exe version
restic 0.18.0 compiled with go1.23.6 on windows/amd64
$ busybox du -h restic_0.18.0_windows_amd64.exe
39.9M restic_0.18.0_windows_amd64.exe
$ busybox du -h restic_0.18.0_windows_amd64_official.exe
29.1M restic_0.18.0_windows_amd64_official.exe
$ busybox du -h restic_0.18.0_disablegrpcmodules.exe
28.3M restic_0.18.0_disablegrpcmodules.exe