There are two overloaded versions of SearchBuf in System.StrUtils:
{$IFNDEF NEXTGEN}
function SearchBuf(Buf: PAnsiChar; BufLen: Integer; SelStart, SelLength: Integer;
SearchString: AnsiString; Options: TStringSearchOptions = [soDown]): PAnsiChar; overload; deprecated 'Moved to the AnsiStrings unit';
{$ENDIF}
{$IFDEF UNICODE}
function SearchBuf(Buf: PChar; BufLen: Integer; SelStart, SelLength: Integer;
SearchString: String; Options: TStringSearchOptions = [soDown]): PChar; overload;
{$ENDIF}
you can see that the deprecated one is the ANSI version., actually the deprecated keyword is to alert you that the function is moved (rather than taken off).
The Unicode version is still there, I think it is more suitable than the ANSI version when working with strings in general.
Remove any reference to the AnsiStrings from your uses section , Keep only StrUtils, and replace AnsiString type with the string type, PAnsiChar with the PChar so that the
compiler will use the Unicode overload instead of the deprecated ANSI overload.