Representatives from Riot argue that a Practice mode is unnecessary for Teamfight Tactics.
If you're a League of Legends player, you're likely familiar with the Practice mode, which allows you to test new champion skills, items, etc. without bothering anyone. Even when you get a new skin, Practice mode is where players can try it out and evaluate if it suits their champion.

The popularity of Practice mode in League of Legends has led some Teamfight Tactics players to question why this game doesn't have a similar mode. Even Riot Mortdog, the Lead Designer of Teamfight Tactics, acknowledged this question in a recent stream. According to Riot Mortdog, Teamfight Tactics doesn't need a Practice mode, as it could potentially harm the game.

Riot Mortdog: 'We believe that a Practice mode is unnecessary and it could ruin Teamfight Tactics. If there was a Practice mode, players would just go there and try every possible way to get a 5-cost 3-star champion without bothering to play ranked anymore. This wouldn't be healthy at all, so the development team at Riot is not creating a similar mode.'
In fact, on the PBE server, Riot Games has a mode with similar functionality to Practice mode called 1v0. Here, players can freely use gold to build their team composition, but there are no opponents for the champions to actually fight against. This mode is created for players on the testing server to get familiar with the champions of the new season, at a time when players on the testing server are most active.

However, this 1v0 mode quickly turned into a virtual world, similar to what Riot Mortdog mentioned, where players only roll for 5-cost 3-star champions. That's why Riot decided not to implement it on the official server, as they want players to enjoy the game through real matches rather than such virtual living.

Basically, Riot Games is sticking to its decision as Teamfight Tactics thrives into its 7th season without the need for a Practice mode like League of Legends. For a highly interactive game like Teamfight Tactics, direct in-game testing is the best approach.