Deoxys has been one of the hardest Pokémon to obtain for awhile. Outside of event distribution and hacking, Deoxys' only reliable appearance was in the Delta Episode, a full 3 generations after its introduction. And it has been two generations since. However, Deoxys' presence is always felt in games where it is available. Random Meteors lying around or in the hands of friendly scientists, always waiting for the rare or lucky trainer to have this legendary alien.
And it is a shame that Deoxys is so restricted because even nowadays, it is outclassed. Too many Pokémon outclass or eat Deoxys forms alive. Deoxys Attack is outmatched by Zacian-Crowned, who, in return for specializing in one offensive stat, has higher effective attack, only slightly slower speed and far, FAR superior defenses and typing. Deoxys Defense is outclassed by Solgaleo and Necrozma-DM who possess a superior typing on top of reliable recovery. And Deoxys Speed has been dethroned by Regieleki as the fastest Pokémon in the game. While yes, it is true that Regieleki cannot replicate Deoxys Speed's niches, the fact that Deoxys Speed actually has something that can outspeed it is a huge blow. And even then, priority is EVERYWHERE these days, so Deoxys Speed's peerless Speed is no longer the shield it once was.
Living Speed
Deoxys-Speed
-Stealth Rock
-Spikes
-U-Turn
-Magic Coat/Taunt
Ability: Pressure
Item: Focus Sash
Nature: Timid
EVs: 252 HP / 4 DEF / 252 SPE
The one niche Regieleki can't take from Deoxys Speed is the suicide lead. Setup entry hazards and die, enabling Deoxys Speed's teammates to muscle past checks that could normally stop them, punish switches, etc. However, Suicide Lead is so weak these days thanks to the prevalence of Defog -it is much more important for setters to have resilience -able to setup multiple times throughout a game. Deoxys Speed doesn't have that. What it does have is peerless Speed that denies near any attempt at disruption while it sets up entry hazards. So if your opponent is relying on Taunt to stop entry hazards, they may be out of luck. U-Turn gives Deoxys Speed some staying power, allow it to get the heck out of dodge while scouting for switches. Magic Coat and Taunt round out the set. Magic Coat punishes other entry hazard setters, while Taunt stops Defog cold.
Oh I forgot to mention Deoxys Normal? Deoxys Normal is outclassed by Deoxys Attack. Literally if you are using Deoxys Normal, 99 times out of 100, Deoxys Attack would suit you better.