The UN and IMF both include China in their lists of developing economies as of 2018. The WTO has no official list of developed/developing countries, it's up to member nations to announce that they are developing countries and it is up to other members to challenge those members' attempts to use provisions offered to developing countries.
Here is a UN report on the world economy, and China is referred to as a developing economy throughout it (such as the table of economic growth on page 3). The Wikipedia page on developing countries mentions the IMF list of developing economies for 2018 (with a citation that leads to a direct download of their report), on which China is included.
As for the WTO, they have the following to say about developing countries under WTO rules:
There are no WTO definitions of “developed” and “developing” countries. Members announce for themselves whether they are “developed” or “developing” countries. However, other members can challenge the decision of a member to make use of provisions available to developing countries.
That a WTO member announces itself as a developing country does not automatically mean that it will benefit from the unilateral preference schemes of some of the developed country members such as the Generalized System of Preferences (GSP). In practice, it is the preference giving country which decides the list of developing countries that will benefit from the preferences.