user4399: Obviously NOT counting draws there are a lot eg Nepo lost 4-0 total to Magnus in the 2021 WCC so removing the draws, there's that.
In the 2019 WFRCC, which was indeed classical, Magnus Carlsen lost to Wesley So in 3 classical games in a row, COUNTING draws: Draw and then 3 losses (and then fast rapid which was draw and then 1 loss).
Did this ever happen in regular classical chess (that are 1v1 tournaments but including finals or tiebreakers of tournaments that aren't 1v1) - 3 losses in a row?
Let's see...

2006-2021 - None.
FIDE: 1993-2004 - Oh just 2 Alexei Shirov vs Vishy Anand in 2000 but was it classical time controls? And there's Anatoly Karpov vs Gata Kamsky in 1993 - ok so WFRCC 2019 was the 1st time since either 2000 or 1993 that someone lost 3 classical games in a row in either WCC or WFRCC.
Classical (PCA world championship i mean) : 1993-2004 - None.

And then before 1993...

user4399: 1930 - 1992:

1986 Garry Kasparov vs Anatoly Karpov
1961 Mikhail Botvinnik vs Mikhail Tal
1958 Mikhail Botvinnik vs Vasily Smyslov
1954 Mikhail Botvinnik vs Vasily Smyslov (again)
1937 Alexander Alekhine  vs Max Euwe

1886 - 1929:

5 in a row 1910 Emanuel Lasker vs David Janowski
4 in a row https://en.wikipedia.org/wiki/World_Chess_Championship_1907
4 in a row https://en.wikipedia.org/wiki/World_Chess_Championship_1896%E2%80%931897
5 in a row https://en.wikipedia.org/wiki/World_Chess_Championship_1894
3 in a row twice https://en.wikipedia.org/wiki/World_Chess_Championship_1889
4 in a row https://en.wikipedia.org/wiki/World_Chess_Championship_1886


4399: <p>Obviously NOT counting draws there are a lot eg Nepo lost 4-0 total to Magnus in the 2021 WCC so removing the draws, there's that.</p>
<p>In the 2019 WFRCC, which was <a href="https://chess.stackexchange.com/questions/41499/why-was-the-2019-wfrcc-called-slow-rapid-when-its-actually-classical-4515-6">indeed classical</a>, Magnus Carlsen lost to Wesley So <a href="https://chess.stackexchange.com/questions/41508/when-was-the-last-time-magnus-carlsen-lost-3-classical-games-in-a-row-besides-to">in 3 classical games in a row, COUNTING draws</a>: Draw and then 3 losses (and then fast rapid which was draw and then 1 loss).</p>
<p>Did this ever happen in regular classical chess (that are <a href="https://chess.stackexchange.com/questions/41483/why-didnt-wesley-so-get-full-championship-privilege-in-the-2022-wfrcc-unlike-ma">1v1 tournaments</a> but including finals or tiebreakers of tournaments that aren't 1v1) - 3 losses in a row?</p>
<p>Let's see...</p>
<ol>
<li>2006-2021 - None.</li>
<li>FIDE: 1993-2004 - Oh just 2 Alexei Shirov vs Vishy Anand in 2000 <strong><a href="https://chess.stackexchange.com/questions/41614/was-the-alexei-shirov-vs-vishy-anand-match-of-the-finals-of-the-2000-fide-wc-in">but was it classical time controls</a>?</strong> And there's Anatoly Karpov vs Gata Kamsky in 1993 - ok so <strong>WFRCC 2019 was the 1st time since either 2000 or 1993 that someone lost 3 classical games in a row in either WCC or WFRCC.</strong></li>
<li><a href="https://en.wikipedia.org/wiki/Professional_Chess_Association" rel="nofollow noreferrer">Classical (PCA world championship i mean</a>) : 1993-2004 - None.</li>
</ol>
<p>And then before 1993...</p>
 4399: <p>1930 - 1992:</p>
<ol>
<li>1986 Garry Kasparov vs Anatoly Karpov</li>
<li>1961 Mikhail Botvinnik vs Mikhail Tal</li>
<li>1958 Mikhail Botvinnik vs Vasily Smyslov</li>
<li>1954 Mikhail Botvinnik vs Vasily Smyslov (again)</li>
<li>1937 Alexander Alekhine  vs Max Euwe</li>
</ol>
<p>1886 - 1929:</p>
<ol start="6">
<li>5 in a row 1910 Emanuel Lasker vs David Janowski</li>
<li>4 in a row <a href="https://en.wikipedia.org/wiki/World_Chess_Championship_1907" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/World_Chess_Championship_1907</a></li>
<li>4 in a row <a href="https://en.wikipedia.org/wiki/World_Chess_Championship_1896%E2%80%931897" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/World_Chess_Championship_1896%E2%80%931897</a></li>
<li>5 in a row <a href="https://en.wikipedia.org/wiki/World_Chess_Championship_1894" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/World_Chess_Championship_1894</a></li>
<li>3 in a row <strong>twice</strong> <a href="https://en.wikipedia.org/wiki/World_Chess_Championship_1889" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/World_Chess_Championship_1889</a></li>
<li>4 in a row <a href="https://en.wikipedia.org/wiki/World_Chess_Championship_1886" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/World_Chess_Championship_1886</a></li>
</ol>
