So I am a huge fan of byo-yomi however a player who has many ko threats which will never be used if there is no ko and the player is sure there will not be a ko (they entered endgame) can use this as an advantage to gain a lot of time.

10 ko threats on 30 second byo yomi periods can be used to gain almost 5 minutes of additional time.

Do you think using this kind of tactics is shameful or is simply a part of the game since surely someone who is reviewing the game and can't see the time played will be confused as to why those moves were played.

Also a side question, was there ever a game recorded where a player played a timesuji during the game only to lose the game because of lack of ko threats later in the game?

To regard timesujis as shameful requires a level of creativity I am unable to muster.

By the way, the equivalent in chess (repeating moves when possible to gain time on the clock) is a time-honored tradition that I have not heard anyone decry.

I can't remember if I've made a mistake so subtle as a timesuji that wasted a ko threat which meant I lost a ko and lost the game, but I have certainly played retarded moves like a gote move I thought was sente as timesuji. In fact I lost my 2nd British Championship game because I played a locally sente timesuji to try to get more time to read a complicated fight, and my opponent wisely ignored it to punish me in the fight which was more important.

It is certainly legal and so cannot be criticized. Whether is it ethical is a different matter and I am on the fence on that.

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Still officially AGA 5d but I play so irregularly these days that I am probably only 3d or 4d over the board (but hopefully still 5d in terms of knowledge, theory and the ability to contribute).

Plenty of things that are legal can be criticized! The subject of sportsmanship is all about what legal actions should nevertheless be avoided.

Nevertheless it is hard for me to see what there is to criticize about playing small moves with large followups when low on time. It seems to me to be a fundamental consequence of the byo-yomi system.

I am not at all sure why you would think sacrifice of forcing moves in order to gain time in byo-yomi to count or read something out "shameful". Nor do I understand why you think a review wouldn't mention the situation, because I have seen plenty of reviews of pro games doing exactly that.

Such time tactics happen even in the Japanese TV pro tournaments I've seen. In those tournaments byoyomi is 30 seconds per move and it is often seen that one player who wants to think about the situation will play a forcing move to gain an extra 30 seconds.

Time tactics are obviously something that goes with byoyomi, and byoyomi is more common in shorter time limit games. The necessity of using these tactics also implies that it is difficult for players to play their best go in short time limit games.

A famous time tactic happened in Shusai's retirement game with Kitani. At one point in that game Kitani sealed his move before an overnight adjournment and he played a forcing move which some commentators said was to give him more time for his next move. There was also a political aspect since Shusai was known for choosing adjournment timing to favor himself.

Time tactics are almost entirely what fast chess games (e.g. 5 minutes per side sudden death) are about. If you watch such games on a chess server you may see a player sacrifice almost all his pieces, hoping the opponent will run out of time.

Actually I own a book called territorial styles of Kitani Minoru and Cho Chikun and that game is described in the book.

However while they did think he played a move like that only to get more time during the night, the commentary shows that the move was actually played with correct timing and was not just a timesuji.

However Shusai didn't really think about it during the game and only became frustrated.

If the record of a game does not include times, then when reviewing the game record one cannot tell whether time was a factor in a move choice (although of course one can guess).

By the way, I am very glad that SGF includes explicit properties for time remaining and that servers like KGS use it. Time spent is an important part of a game record, both for educational purposes (how long did Black spend thinking about this position?) and informational (how much time did Black have to think about this position if he wanted?), and it's a real shame when it is thrown away (as often happens in chess).