I'm desperate enough at this point to start applying for more junior roles. How should I respond when I inevitably get asked during an interview, "Why are you applying for a Junior Developer role when your last position was Senior Developer and you have 20 years of experience?"
The honest answer appears to be something along the lines of "Since getting laid off, I've been unable to land a Senior job, and now I'm getting desperate."
A more nuanced answer might be something like "Since getting laid off, I've realized that despite having the title I'm not really qualified to be a Senior Developer in most organizations. I've decided to take a step back and grow my career a bit." Only state this if it is true. Discuss how important it is for you to improve your skills in your next job.
Be ready to answer questions about how long you can expect to be happy in a role that is less than Senior. And be ready to talk about the salary you may be asked to accept. Hiring managers will likely be wary of someone who would go from Senior to Junior - they may expect them to jump as soon as a position becomes available. You may need to calm those fears in order to get hired.