Actually, I think that it is, if it supports UTF-8 without BOM and does not try to do such things like convert quotation marks into non-ASCII quotation marks automatically, etc.
However, using a proper ASCII text editor would be better, if you do not want Unicode, to avoid many of the problems with Unicode if you are loading an unknown file or copying unknown stuff using clipboard, etc, which may be homoglyphs, reverse text direction override, etc.
(I use vim with exclusively ASCII-only mode, and do not use a Unicode locale, on my computer.)
I just mean, if you _require_ plugins in order to be able to edit the content, then the content can't easily be described as text. It is fine to use a specialized application to edit a file of a non-text format, I have nothing against that, but you have then left the realm of text editor.
As an example of what I mean, if someone wrote a vim plugin that allowed a user to interact with a sqlite file and change the schema or edit raw row values from vim, it could be a really valuable and useful plugin. But the presence or absence of a plugin for some given text editor doesn't change whether a given format is generally considered a format suitable for being edited in a text editor. What it does instead is convert vim into an editor of non-text files.
I admit that the proposed file format is much closer to being editable in a text editor than a binary format such as sqlite, but the fact that the characters cannot be typed without special functionality suggests that the format is not suitable for a text editor.
The described format _is_ editable in vim without a plugin. It's just a little awkward, because everything will be on a single line, and you have to use more complicated key commands to enter the delimiters (for example `<C-v><C-6>` for \x1e).
You just need a text editor that can support thia format.