In Cyrillic typography tradition you need to duplicate the math operators (+, =, -, x) on line break. Following this answer I can get it working in math mode. How can I make = duplicate on line break in text mode:
MWE:
\documentclass{article}
\usepackage{amsmath}
\mathchardef\mathequals=\mathcode`=
\begingroup\lccode`~=`=
\lowercase{\endgroup\def~}{\mathequals\discretionary{}{\the\textfont0=}{}}
\AtBeginDocument{\mathcode`=="8000 }
\mathchardef\mathplus=\mathcode`+
\begingroup\lccode`~+`+
\lowercase{\endgroup\def~}{\mathplus\discretionary{}{\the\textfont0+}{}}
\AtBeginDocument{\mathcode`+="8000 }
\begin{document}
In math mode: $a = b = c = d = e = f = g = h = i = j = k = l = m = n = o =
p = q = r = s = t = u = v = w = x = y = z + a + b + c + d + e + f + g + h +
i + j + k + l + m + n + o + p + q + r + s + t + u + v + w + x + y + z$
In Text mode: a = b = c = d = e = f = g = h = i = j = k = l = m = n = o =
p = q = r = s = t = u = v = w = x = y = z + a + b + c + d + e + f + g + h + i
+ j + k + l + m + n + o + p + q + r + s + t + u + v + w + x + y + z
\end{document}


\discretionary{=}{=}{=}– David Carlisle Apr 08 '19 at 18:14