According to the following links, it is commonly pronounced "Cecil".
https://kaufmanrossin.com/blog/bank-ready-meet-cecil/
The proposed model introduces the concept of shifting from an incurred
loss model to the current expected credit loss model commonly referred
to as CECL (pronounced “Cecil”).
http://www.gonzobanker.com/2016/02/cecl-the-blind-leading-the-blurry/
[...] and its name is CECL (Current Estimated Credit Losses, pronounced like
the name “Cecil”).
The name Cecil means “blind,” which is ironic, because FASB’s upcoming
guidance will push FIs to clarify the future performance of their loan
portfolios by using models to predict CECL of all loan portfolios.
https://www.linkedin.com/pulse/operational-financial-impact-cecl-banks-nikhil-deshmukh
Termed as Current Expected Credit Loss (CECL, or Cecil, as some call
it), [...]