I literally LOL’d 
And yes, Tahoe 26.0 is a bigger mess than expected.
Hello!
This issue is certainly very annoying haha. To download a new installer and overwriting it works but have done that three times now. The extension just disappear. I know this is not on you, but on apple. Hopefully there is a fix soon. Regret going with Tahoe this early (never happens usually).
Did you try the Terminal commands (see above)? In addition, the extension is not really required, there are other options:
Yes and it worked after a restart of safari. Hopefully it stays where it should be from now. The other paths is noted 
I confirm this worked.
For the record, I’m not against using Voodoo for solving twilight zone issues like this one. But, what dark forces have been at work here when we called upon them with “pluginkit -e”?
Anyone having issues… do you have Share across devices enabled in Safari’s Settings > Extensions?
Bingo! After disabling that option the sharing extension is back in Safari. I will see if it “sticks” and let you know tomorrow.
I am not sure if it’s related yet.
If you are so inclined and are having an issue with the extension disappearing after a machine reboot, then yes, please disable it, get the extension set up again, and see if it persists after another reboot. Totally optional thoughl
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Will let you know tomorrow morning!
Much appreciated 
Yes.
Both extensions (share and safari) have disappeared since my last reboot.
Do you have Share across devices enabled in Safari’s Settings > Extensions?
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Unfortunately the extension is missing again. When I started up my computer and Safari the extension was showing in the toolbar for maybe 5 seconds and then disappeared. It doesn’t show in Safari > Safari Extensions right now. In System Settings > Login Items & Extensions only the Spotlight extension is listed.
Thanks for checking!
Could share across devices here be a problem? (I have done the “install dance” again so the extension is currently working.)
Not sure yet. Just trying to find potential causes. I’ll look more when i wake up as it’s almost 2am here 
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et al:
If the extension disappears in Tahoe again, quit and relaunch DEVONthink, then check Safari’s Settings > Extensions. Is our extension visible again?
Not for me. I have Share across devices unchecked, but the extension still disappears with each reboot, and quitting DT (and Safari) doesn’t bring it back in Safari’s Extensions. No problems with going via the Sorter.
Thanks for checking!
An update. When I disabled Sharing across device as you suggested the Safari extension didn’t stay enabled after reboot. I then enabled the extension again but didn’t bother to reenable Sharing across devices. Now the DT Safari extension has remained after reboot twice. So maybe there is something to your hunch after all.
Could this procedure be a workaround? 1. Disable Sharing across devices. 2. Reboot 3. Enable DT Safari extension.
However, reading @NickLowe‘s posts here I realize that it is unlikely to fix it for everyone.