Hi, I'm not on the t5 Gemma team but work on gemma in general.

Encoder Decoder comes from the original transformers implementation way back in 2017. If you look at figure 1 you'll see what the first transformer ever looked like.

Since that time different implementations of transformers use either just the encoder portion, or the decoder portion, or both. Its a deep topic so hard to summarize here, but Gemini explains it really well! Hope this gets you started on some prompting to learn more

https://arxiv.org/pdf/1706.03762