When BTS stopped accepting gifts from fans
January 24, 2018

In BTS's early years, sending gifts to the members was a completely normal part of fandom culture. For birthdays, anniversaries, and other special occasions, packages arrived, along with clothes, food, and all sorts of things fans picked out for individual members. As BTS grew more popular, so did the number of fans who wanted to send something, and with it the volume of gifts arriving at Big Hit.
In early 2018, the company decided to change the rules. On January 24, Big Hit Entertainment announced on the official fan cafe that BTS would no longer accept gifts and organized fan support the way they had before. A transition period was left for projects that had already been arranged, and from March 30, 2018, only letters could be sent to the members.
The decision wasn't the result of any known incident, nor a sign that BTS no longer wanted that kind of contact with fans. The members had been receiving gifts and thanking the people behind them for years already, but they now felt they'd received more than enough. Messages from that period also show some discomfort with the time and money fans were spending on them.
j-hope thanked fans at the time for everything they'd sent him over the years and explained that he felt it was now his turn to be the one giving something to them. The other members held a similar view. They weren't rejecting the care behind the gifts, just the need for fans to spend money on them in the first place.
The timing of the decision makes sense too. By early 2018, BTS no longer had the small fandom of their early career years. Fans were spread across the whole world, and birthday and other fan projects kept getting bigger. In K-pop, fan support didn't necessarily mean just a few plush toys or a box of sweets. Popular idols were sent expensive fashion items, electronics, huge quantities of food, and joint gifts funded by entire fan clubs.
The rule introduced in 2018 still stands today. In the meantime, a large part of the communication between BTS and ARMY has moved to Weverse, and physical gifts to the members are still not accepted. Fans can, however, still write to them. Today the simplest official way is Weverse's Fan Letter feature, which lets you send a digital letter to an individual member or to the whole group.
So a practice that was completely ordinary in BTS's early years ended right at the point when the size of the fandom was making it harder and harder to sustain. The gifts disappeared, but the ability for a fan to send the members a personal message has remained, eight years later.
Sources: official Big Hit Entertainment notice on fan gifts, January 2018 · BIGHIT MUSIC · BTS Weverse Fan Club Etiquette · Weverse Fan Letter · Korea JoongAng Daily · Yonhap News Agency · archival messages from BTS members to fans
Unofficial fan site. Photo used under the stated license, with credit to the author.