"A custom game where one can play solo. To discover the maps."
I spent the first two years of the game wishing for exactly this. My guess is that they want you to discover the maps only through live gameplay because it's good for their metrics. I'm honestly surprised they were willing to introduce Bot Royale, and even more surprised that they haven't scrubbed it yet.
"But a game mode just for shooting, solo like in an arena."
I'm also surprised they haven't introduced a rumble-style game mode. They could easily do it on the firing range. Say, four to eight players, no timer, just a "first player to 30 kills wins" mechanic. Like you, I suspect a lot of people would love it. At any rate, they've given us solo battle royale twice now (though they were both flawed in different ways) so maybe at some point that will return.