i deletled a mod in my modpack, but the dimlets related to this mod are still here

You means the ones you already have in chests and inventories? In that case that's hard to avoid. There is no way to automatically delete these

Thank you for taking the time to create this issue, as we're no longer supporting versions below 1.12, I'm going to go ahead and close it but if this bug seems to continue over to newer versions, please do report it again so we can investigate the issue further.