It's a good question but my answer would be that it's still security by obscurity because if you know that Caesar Cipher is used, bruteforce indeed is required, but the search space is so low and there are no serious consequences if you bruteforce.

Now how is this different from VIP lemo/decoys or dead-drops, you may ask?

In case of a dead-drop, if you notice, I had mentioned explicitly that the search space must be huge. Else that too is not good - because someone can find your dead-drops easily. And large search space just means that you need to increase the time for others to find it by bruteforce so that by then, you are done with the task :)

In case of VIP lemo/decoys, the search space might be small - maybe 3-4 decoys. But if an attacker wants to target the VIP, he will target 1 of the decoys first and alert the VIP and his team, and hopefully get caught.

So if the search-space is sufficiently large or if with a small search space, the consequences of making even a single attempt are drastic, then I would say the security measure is quite good and it's not security by obscurity.

With Caesar Cipher, I agree that you need to bruteforce the key, but do you think there is any consequence of me having to search that 26 or 52 or even 255 characters for that matter?

Neither would this alert the enemy, nor would it be large enough to hold me from reading the message for a longer time.

I hope that this answers your question :)

And thank you for the kind words! Really appreciate it :)

Cheers!