Thanks Tobias and Mat,
The question has arisen in a Facebook conversation in a group Dmitry and I happen to belong to. The group is a "systems thinking" group, I have been playing with TW and systems (whatever they may be) in partnership. TW can be seen as is a system, a technological system and a social one. TW is my muse!
The question about saving a read only version is one which has occupied my mind before.
When I (with huge amounts of help from Tobias and Eric) produced OMM:Director[2] on TiddlyWiki Classic, I ended up creating a read only site by using jQuery in Firefox's developer tools to show the editing controls. And of course I can't remember how to do that now. It was a wearying thing at the time (TiddlyFans will remember with fondness Jeremy's "Jake Weary" quip) but wrestling with the problem over a long period of time has left its mark. Ah-Ah! Learning!
Its a "personal learning story" I suppose, and as such it may or may not be "true". But the output is an idea: to change a system you can't use that system, there has to be something -- a method, a way of doing things -- outside that system.
(In my mind there is a link here to Godel, and GEB, Hofstader and the quine, indirect self reference and all that: Like I say, the learning story, and learning narratives in general, come with the risk of error, especially when there are leaps of faith, over grounds where the detail is way beyond my theoretical understanding.)
With TW5, there are "outside the system" options. Node.js for starters. I thought that there could be a command line option to create a read only TW from a collection of tiddlers.
The standard TW file itself does have ways of exporting content without editing features.
Individual tiddlers can be downloaded or "exported"

Screenshot showing export options
Another option is through advanced search and filters

Sreeenshot showing export via filter
Towards possible solutions
Similar to export Static HTML, a TiddlyWiki file is exported
Node.js, something like this
tiddlywiki mynewwiki --ReadOnly
A read only version of a HTML file would be created on your local hard-drive, could could then share it as you wish (host on GitHub, send by email etc.)
Hacky way, adapt existing
I looked at the ReadOnlyTheme
Screenshot: inside ReadOnly Theme tiddler
The tiddler has a rule pragma. Its JSON, there is a list of buttons to hide. Removing the save button from the list and saving as a new theme creates a read only with save button theme.
I switched themes downloaded the TW (changed the name of the file to ReadOnlyWithSave.html) to my desktop, then uploaded to my GitHub pages [3]
This "hacky way" removes the editing features from the TW but does not remove the possibility of editing.
Future hacky ways?
Various ways of hiding the save button could be explored. creating a save button in a hidden-from-menus tiddler, then hiding the save button from the sidebar would be one option
best wishes
Alex