Not entirely true. If the DM has run enough mods to complete a specific "DM Quest" for the season the mod is from, they are allowed to add a minor magic item to the adventure - and one of the options is a +1 weapon. I've seen a couple of +1 halberds handed out this way.
Plus, when running one of the hardback book adventures, you frequently roll for random treasure, which may result in a magical polearm. Not something I'd count on, but DM's have been known to "adjust" the random rolls. And a very few mods give the players a choice. (White Plume Mountain from Tales of the Yawning Portal gives an option for a +2 weapon of your choice from the NPC who hired you if you successfully complete the adventure. I have a character with a +2 hand crossbow as a result of that, and another with a +2 quarterstaff)
But there are more reliable methods. I have a Hexblade Pact of the Blade Warlock who uses a Halberd (reskinned as a large scythe) for her pact weapon, and with Improved Pact Weapon it's a +1 Halberd for her. (For her the Hexblade is literally the Grim Reaper's scythe...)