The changes in CF7 5.7.3 make it no longer viable to support the old format of the [multistep] tag. Previously, even if a form tag supported a name it would be a valid tag without a name. In CF7 5.7.3, these types of tags are ignored.
The name in the tag was being used to determine the old vs the new format of the form tag, but now that the name is required, this is no longer possible. If you see the [multistep] tag in your forms, please make sure you are using the “new” format which has been the main format for 3 years now.
The old format looks like this:
[multistep "1-3-/your-next-url/"]
The new format looks like this:
[multistep multistep-123 first_step "/your-next-url/"]
Please see this post for additional information:
https://webheadcoder.com/contact-form-7-multi-step-forms-update-4-0/
In a related issue, the [multiform] tag has been fixed in the latest version (4.2) of this plugin.