If you're returning to Pokemon Go or still hold some of your 2016 Pokemon, then under no circumstances transfer them out as they could help you immensely.
Pokemon Go released in July 2016 and it seems Niantic are rewarding the players who played it upon release. From September 2018 onwards, the game developers introduced a new feature that would guarantee a lucky Pokemon when trading under certain conditions.
If you trade Pokemon and adhere to the following you are guaranteed a lucky Pokemon:
You can do this until you reach the limit of 10 whereby they may still be lucky but won't be guaranteed. All traded Pokemon have the chance of being lucky but the older the Pokemon are, the higher chance this will be. It should also be noted once you trade a particular Pokemon that it can no longer be traded again.
For those who don't know lucky Pokemon have an IV floor of 12/12/12 meaning each of their attack, defense and stamina values will be a minimum of 12. They also cost 50% less stardust to power up - a very useful addition if you want to power it up beyond level 30.
The floor of 12 across the board also increases the likelihood of getting a perfect IV Pokemon, too. The odds jump massively to just a one in 64 chance.
2016 Pokemon don't just guarantee high IVs and a power up discount though. Some of these Pokemon may have legacy moves which aren't available today.
You can look for legacy moves using IV checker apps such as PokeGenie. They are particularly powerful hence the reason they aren't available now.
According to postgiven's guide on Reddit they "are wanted by people if they are still under 1500 Cp for PVP (18)". They go on to say: "If you evolve the Pokemon it'll lose the Legacy moves and if you level it too high it won't go down to 1500 when traded. So don't level or evolve them especially if they are Legacy."
Additionally, 2016 Pokemon cannot be evolved to learn legacy moves and as stated above will actually lose the move if they are.
In short, if you have a 2016 Pokemon, do not transfer them - learn about their potential and make the most of them because they're actually pretty valuable.
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