LogFAQs > #972098196

tl;dr Boktai stuff is in

https://news.capcomusa.com/2023/03/09/connect-with-the-latest-mega-man-battle-network-legacy-collection-info/

With permission from Konami, the Boktai collaboration contents have been included in this collection! Unfortunately, crossover battles with the Boktai series wont be possible. The games in Mega Man Battle Network Legacy Collection are based on the original Japanese games on Game Boy Advance that required a physical connection to the Boktai series games to access collaboration contents. That said, the Boktai collaboration Battle Chips, newly translated stories, and Patch Cards will be available! These collaborations are available in Mega Man Battle Network 4, 5, and 6.
Long-time fans are already aware, but to get others up to speed, this content was not available in Western releases of Mega Man Battle Network games. This marks the first time that some of this content will be made available in territories outside Japan!