There is some debate as to whether or not the Trump tweet constitutes the crime of witness intimidation. However, Yovanovitch did say she felt intimidated by the tweet.
Some argue that she wouldn't have know had not Schiff brought it to her attention. However, the counter to that is that a tweet of disparagement about anyone coming from the president bears the full weight and power of the presidency and there is a more than reasonable expectation that that tweet during that testimony would be immediately made known to her for obvious reasons. Therefore, the "but for" argument is rather meaningless.
Therefore, in my view, despite arguments to the contrary, Trump committed the crime of witness intimidation because:
1. She said she was intimidated by the tweet
2. Given that it's coming from the president while she was bearing witness, given the more than reasonable expectation she would be made aware of it,
it is therefore witness intimidation.
That being said, I am no legal expert, not a lawyer, and though I could be wrong, I like this particular point of view:
Diane Marie Amann, law professor, University of Georgia
Impeachment is a legal process, in that the US Constitution provides for charging of “high crimes or misdemeanors” by the House, followed by trial in the Senate. Impeachment is not a federal criminal proceeding, however.
For this reason, evaluation of the president’s statements — in tweets, in phone conversations, at press gaggles, and the like — should turn not on whether the precise elements of a federal crime like witness tampering have been met, but rather on whether the statements constitute, or contribute to, abuses of the public trust that justify the president’s impeachment and removal from office.