Just to save your time, I had issues accessing my Minio S3 repository behind a http/https proxy and I didn’t find anything on Restic docs, so:
RESTIC_REPOSITORY=s3:https://example.com/my-bucket
On powershell:
$env:https_proxy="http://username:password@1.2.3.4:8080"
On command prompt:
set https_proxy=http://username:password@1.2.3.4:8080
Hope doing something useful.