The final release of the original MUSCLE software package was v3.8.1551, posted July 12th 2014. Most of the code was developed in 2003-4 when most PCs had a single CPU core with 256 to 512 Mb RAM.

In 2021, the code was re-written from scratch for MUSCLE v5 to implement novel algorithms and fully support modern PC hardware.

 
MUSCLE v3 paper
Edgar, Robert C. "Muscle: multiple sequence alignment with high accuracy
  and high throughput"
📝 Nucleic acids research 32.5 (2004): 1792-1797