How immutable do you need?
S3 offers a flag that prevents modification or deletion for a set period, and afaik basically every S3-compatible provider offers that.
I use that along with a lifecycle rule to maintain my backup buckets on iDrive.
If you need a 'you cannot touch this and the provider has no way of allowing it' then you're talking specialized corporate talk-to-a-sales-person-for-a-quote, as you found out.
Edit: if you don't need cloud, there's options for WORM media from the humble BluRay to fancy SSDs that don't allow deletion.