Just because something can be a profitable method for a discretionary trader doesn't mean that its anywhere near optimal or how the big players are playing the game.
As a long time retail discretionary trader, I don't care what the "big players" say. I have no interest in running a fund nor working for one nor how they run them.
The most obvious horizontal trendlines are those at the (previous bars) Open, High, Low, and Close. Quant fund human resource departments begin to drool and melt the moment anyone begins suggesting novel methods for using horizontal trendlines, as these horizontal trendlines form the core from which all Quantitative Analyisis follows.
TL's can be at any angle including horizontal. If trend is flat then it's flat, doesn't happen often but can't be ruled out. Most likely found in shorter term trading than longer term. Longer term horixontal trend lines can be found where a company is under a buy out offer.
Trend lines can be up/down sloping, horizontal, or curved. This is a hard fact written about since the 20's - not going to debate with idiots on this this subject.
Moving-average crossovers have been around forever also and everyone knows how well they perform when running a back-test. Reckon this thread is conclusive evidence that the 'like' system in this forum is being manipulated by a minority and in need of serious revision or a disclaimer. e.g. "Likes are for entertainment purposes only and in now way represents a credible endorsement or the correctness of each post."
AS Xela and comagnum mentioned, the term 'horizontal trend line/channel' does exist as found in well known books on TA. Attached are examples from two such books.
Logic today is a lot more advanced than it was back then that's for sure, it's like comparing the Ford Model A to the Tesla Model 3.