user-1: I have played a couple of games as black with this line. It is a simple plan: to put pressure on the weak c3 and c4 pawns.
This line is mainly known as the Dzindzi-Indian Opening, according to the comments and this video. It's also called the Modern Defense: Pterodactyl, Beefeater Variation according to chess.com.
[FEN ""]

1. d4 g6 2. c4 Bg7 3. Nc3 c5 4. d5 Bxc3 5. bxc3 f5

I have tried it while playing correspondence, so I have followed the move suggested by databases: 5. f5. This has had good results after quickly playing Qa5, trying to attack the weak pawns along with other minor pieces.
5...f5  347
2018    2403/2426/2709   30.3%  25.6%   44.1%
5...Qa5 22
2017    2359/2306/2586   50%    22.7%   27.3%
5...e5  5
2014    2370/2338/2418   40%       20%     40%
5...d6  1
2010    Steadman vs. Bosboom 0-1
*Chesstempo database for the position
I wonder what the reason is for taking away a tempo to play f5 while the plan is clear, at least as I play it: to profit from the doubled pawns by attacking on the queenside.
Is f5 played to avoid e4?

user2174: f5 has the aim to stop the e4 push. However, a "refutation" of the line is 1. d4 g6 2. c4 Bg7 3. Nc3 c5 4. d5 Bxc3 5. bxc3 f5 6. e4 fxe4 7. f3!; Here, if black captures in f3, then white has a strong attack due to the better development. Therefore, in his DVD series on the Dzindzi, GM Ron Henley suggests 7. ...e5!, but he doesn't consider the first choice of Stockfish, which seems very good for white: 8. Nh3! with the idea of Nf2 (ff 8. ... exf3 then simply Qxf3).

user12873: By playing ...f5 you're trying to weaken the e4 push. This is the most important immediate thing to attend to. It's not like White's doubled pawns are going to run away anytime soon.

user14202: You don't want a White pawn getting to e5, especially with a friend at f4.  White can let you have a c-pawn if he can achieve such a central dominance.
Also, without f5 it is awkward to develop the Ng8 -- you notice none of the variations you show have Nf6.  But now after f5, Nf6 will grip the e4 square.   Thus White often plays e4 anyway!

-1: <p>I have played a couple of games as black with this line. It is a simple plan: to put pressure on the weak c3 and c4 pawns.</p>
<p>This line is mainly known as the Dzindzi-Indian Opening, according to the comments and <a href="https://m.youtube.com/watch?v=PHvUGZq3iuo" rel="nofollow noreferrer">this</a> video. It's also called the Modern Defense: Pterodactyl, Beefeater Variation according to <a href="https://www.chess.com/openings/A40-Modern-Defense-Pterodactyl-Beefeater-Variation" rel="nofollow noreferrer">chess.com</a>.</p>
<pre><code>[FEN &quot;&quot;]

1. d4 g6 2. c4 Bg7 3. Nc3 c5 4. d5 Bxc3 5. bxc3 f5
</code></pre>
<p>I have tried it while playing correspondence, so I have followed the move suggested by databases: 5. f5. This has had good results after quickly playing Qa5, trying to attack the weak pawns along with other minor pieces.</p>
<p>5...f5  <strong>347</strong><br />
2018    2403/2426/2709   30.3%  25.6%   44.1%</p>
<p>5...Qa5 <strong>22</strong><br />
2017    2359/2306/2586   50%    22.7%   27.3%</p>
<p>5...e5  <strong>5</strong><br />
2014    2370/2338/2418   40%       20%     40%</p>
<p>5...d6  <strong>1</strong><br />
2010    Steadman vs. Bosboom 0-1</p>
<p>*<a href="https://chesstempo.com/game-database.html" rel="nofollow noreferrer">Chesstempo database for the position</a></p>
<p>I wonder what the reason is for taking away a tempo to play f5 while the plan is clear, at least as I play it: to profit from the doubled pawns by attacking on the queenside.</p>
<p>Is f5 played to avoid e4?</p>
 2174: <p>f5 has the aim to stop the e4 push. However, a "refutation" of the line is 1. d4 g6 2. c4 Bg7 3. Nc3 c5 4. d5 Bxc3 5. bxc3 f5 6. e4 fxe4 7. f3!; Here, if black captures in f3, then white has a strong attack due to the better development. Therefore, in his DVD series on the Dzindzi, GM Ron Henley suggests 7. ...e5!, but he doesn't consider the first choice of Stockfish, which seems very good for white: 8. Nh3! with the idea of Nf2 (ff 8. ... exf3 then simply Qxf3).</p>
 12873: <p>By playing ...f5 you're trying to weaken the e4 push. This is the most important immediate thing to attend to. It's not like White's doubled pawns are going to run away anytime soon.</p>
 14202: <p>You don't want a White pawn getting to e5, especially with a friend at f4.  White can let you have a c-pawn if he can achieve such a central dominance.</p>

<p>Also, without f5 it is awkward to develop the Ng8 -- you notice none of the variations you show have Nf6.  But now after f5, Nf6 will grip the e4 square.   Thus White often plays e4 anyway!</p>
