Are non-accountable travel allowances from my employer considered taxable income?

Generally, yes, more so than an accountable, receipted arrangement. A non-accountable travel allowance is a flat amount your employer pays you without requiring you to submit receipts or otherwise account for your actual expenses — because it isn't tied to demonstrating real costs, it's more likely to be treated as taxable income than an arrangement where you're reimbursed based on actual, documented business expenses.

This sits in contrast with an accountable reimbursement, where amounts are tied to real, substantiated expenses and are generally treated more favourably for tax purposes, since they're genuinely reimbursing a cost you actually incurred for work rather than functioning as extra, unconditional compensation. A flat per diem or travel allowance that you receive regardless of what you actually spent looks a lot more like additional pay than a true expense reimbursement, which is exactly why it tends to get taxed.

If your employer offers you a choice, or is deciding how to structure travel allowances for its workforce, an accountable arrangement tied to actual receipts and business travel generally produces a better tax result than a flat, non-accountable amount. Employees receiving a flat travel allowance should expect it to show up as taxable income and plan accordingly, rather than assuming any amount labelled a "travel allowance" escapes tax.